。
She can run a full marathon.
Literal
She [topic-は] full-marathon [object-を] run [thing-こと] [subject-が] can-do.
~ことができる is the formal/explicit potential construction — literally 'the act of X-ing is possible.' The plain potential 走れる would also work and would feel more conversational. フルマラソン ('full marathon') uses フル as a clarifying prefix to distinguish from ハーフマラソン ('half marathon'); in Japanese marathon culture, the distinction is explicitly marked. The standard 42.195 km full course originates from the 1908 London Olympics — a measurement now universal.